1. Field of the Invention
The present invention generally relates to electronic circuits and, more specifically, to radio frequency transceiver circuits.
The present invention more specifically applies to circuits equipped with a coupler for extracting data relative to the reflection loss on the antenna side and, more generally, relative to a charge impedance at the output of a directional coupler.
2. Discussion of the Related Art
Radio frequency transmit/receive chains generally integrate couplers. A coupler is used to sample part of the power present on a so-called main or primary transmission line towards another so-called coupled or secondary line, located nearby.
Couplers can be distributed in two categories according to whether they are formed of discrete passive components (lumped-element coupler) or of conductive lines which are close to each other to be coupled (distributed coupler). The present invention relates to the second category of couplers.
In many applications, it is needed to sample part of the power transmitted over a line, for example, to control the power of an amplifier in a transmit circuit, to control the linearity of a transmit amplifier according to the loss due to the reflection of an antenna, to dynamically match an antenna, etc. A coupler is used to sample this information.
More and more often, communicating electronic appliances (for example, cell phones or laptop computers) are capable of operating over different frequency ranges (for example, GSM and WLAN). They are then generally equipped with several antennas. Further, such transceiver circuits are capable of operating at the same time. Presently, the antennas are not perfectly isolated from one another. This results in a risk for an antenna to capture the transmissions of another frequency band with a non-negligible level with respect to the desired signal.
This poses a problem in the use of couplers. In particular, in the presence of a directional coupler generally having the function of measuring the reflection loss on the antenna side (and thus a possible mismatch), the level of the parasitic signal originating from another frequency band is capable of being greater than the level of the desired signal, thus corrupting the measurement.
To solve this problem, a current solution is to associate, with the secondary line of the coupler, demodulators of the signals present thereacross. Such a demodulation enables to remove the parasitic frequency band and to properly calculate the reflection loss. However, this demodulation has a high cost.
It would be desirable to improve the reliability of the measurements performed by a coupler at the lowest possible cost.